There's a setting in the terminal-parameters 6400/4600 (or whatever they're called, I have no switch to look it up), if you set "Customize parameters" to yes (if not allready so), you get some extra options, one of them is "Volume". Set it to "retain handset & speaker between calls" and it should work as expected.
Not all terminal parameters apply to IP sets, but this one does. You will have to refresh the station to make it effective (either do a busy & release, or dial the refresh parameters FAC followed by the extension).
Cheers,
Nico